Author Topic: Tennessee mountains and jake brake driving  (Read 10437 times)

Richard Cooper

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Tennessee mountains and jake brake driving
« on: August 02, 2012, 03:22:19 AM »
Tomorrow I'm heading out of Nashville to Chattanooga, Tennessee on I-24 East and have to go over Mounteagle and down a 5% grade over about 5 miles.

Should I adjust the Allison transmission gear to 1 or 2 or 3 besides engaging the full jake brake?  I have in my 2001 Beaver Marquis the Meritor ADB Series Dual air disc brake system w/auto slack adjusters.

What if the jake brake were to fail --- besides pressing the foot pedal brake would I slow the motor coach changing the gears on the Allison transmission settings?  Normally it just shows "6", but automatically adjusts for climbing hills.  I'm talking about going down the hill.

Joel Weiss

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Re: Tennessee mountains and jake brake driving
« Reply #1 on: August 02, 2012, 03:49:06 AM »
Going down hills I find the 4th gear default on my Jake to be satisfactory on all but the steepest grades.  The 5% grade you mentioned will be fine probably with either half or full Jake.  For any particular hill, what I do is decide which Jake setting will permit me to descend without speeding up at my desired speed; you can never have "too much Jake" but it may actually provide so much braking that you have to press on the accelerator occasionally.

On the very steepest grades (~>7%) I will slow my speed to <45 mph and manually shift the Allison to 3rd. I've only had to do this a couple of times on a few long, steep grades.  If you didn't have a Jake you would do the same thing, manually take the transmission down to 3rd or 4th.

Richard Cooper

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Re: Tennessee mountains and jake brake driving
« Reply #2 on: August 02, 2012, 04:22:23 AM »
That is very good info and I thank you.  

I know this question must seem elementary to most everyone reading, but to me it is a new step in my less than 1 year experiences driving a motor coach.

JimCasazze

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Re: Tennessee mountains and jake brake driving
« Reply #3 on: August 02, 2012, 04:27:39 AM »
Be careful of engine RPM and do not allow engine to exceed 2100.  Brake if you must.  Speed at that RPM will vary by coach but on our patriot thunder c12 it is about 63 mph.  I try nit to engage Jake over 60 to be safe.  Low Jake works on up to about 4% with a toad and no toad brakes.  Full Jake holds me on 6 to 7%.  Over that I slow way down and/or use medium brake pressure fo 2-3 seconds ar a time.  This has worked well for us up to one road that was 11% that I should have avoided.

Edward Buker

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Re: Tennessee mountains and jake brake driving
« Reply #4 on: August 02, 2012, 05:00:43 AM »
Joel has given you some good insight. If you find yourself cresting a climb and there is a downhill grade that you are approaching prepare yourself by getting the speed down a bit at the end of the climb, maybe to the 50 -55mph range if you are going faster. If secondary road and steep, even slower of course. This prep gives you a little more time before any downhill acceleration to set the jake on at the appropriate time and gear. 4th gear and 50 to 55mph is a good match on the interstate most of the time. Joel points out that 7% slopes or steeper may warrant third gear and a slower speed. Rather than throttle like Joel mentioned I like to rock the Jake between hi and low postions and off to maintain the appropriate speed. There is no right and wrong to this just a preference. You need to always manage your speed while keeping the engine at 2100RPM or less by adding the service brakes to the jake braking as needed.

If your coach is programmed to go to 4th gear when you select the jake brake you then also need to know the max speed that you can turn it on at and still stay within the engine 2100rpm limit. That speed is likely to be under 65mph. If that is the case you may want to go to an Allison shop at some point and have them change that to a 5th gear selection. The gear that the Jake goes to will be found in the upper right corner of the Silverleaf screen while it is set in the drive position with the jake enabled at highway speed (50-60mph).

Later Ed

Joel Weiss

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Re: Tennessee mountains and jake brake driving
« Reply #5 on: August 02, 2012, 05:50:17 AM »
Quote from: Richard Cooper
I cannot imagine doing 11% grade.  Is there such a thing on USA Interstate Highway System?  Surely not.

I'm going to try to keep my speed at 45 tops and with full jake on.  Just hope a truck behind me doesn't get all antsy.  He can go around.  After all I have to wait behind them going up.

When I was taking my CDL training, I was taught to establish in my mind a safe speed for the grade I am on.  Then brake to ~5 mph below that speed, release the brakes and let the speed slowly creep up.  When it reaches the safe speed again, apply brakes again and reduce.  Doing this cyclically gives your brakes a chance to cool between applications.

This advice is valid whether or not you have a Jake.  If you are using full Jake and are still gaining speed then use the brakes as described.  If the grade is too steep for this approach downshift to a lower gear but you may still have to use your brake as a supplement.  Coming through Lookout Pass on I-90 I was in 3rd following an 18 wheeler at ~45 mph; I only had to use the brakes twice in the entire descent.

As for going down hills and worrying about other drivers, here's an old trucker quote that I like to keep in mind: "I can go down a hill too slow many times, I can only go down it too fast once!"  

As for grades of 11% or more, if you don't like them don't go to Canada.  Going around the Gaspe Peninsula a couple of years ago we got used to 14% grades (up and down) for several kilometers of length.  There's one grade on the Cabot Trail in NS that is ~22%.  We did it in our Class C, but I don't think I want to try it in the Beaver.
 
Lastly, to clarify my approach on grades when the Jake is providing too much braking, I rarely actually have to accelerate, but you will find there is a throttle position at which the Jake's effect is stopped but there is still no significant fuel being provided.  I often shift the Allison to 4th so when I ride this "sweet spot" the transmission doesn't hunt gears, but, rather, just stays in 4th with the Jake being turned on and off as I wish.